restorer: Change endless loop to sys_exit in case of error

If an error happens during late restore stage, we better
exit with error than dive into endless loop.

The loop was useful at early prototype times,
but not now when we use zdtm test suite.

For example, force injected error now reports

 | Restoring EXE (/home/crtools/test/zdtm/live/static/selfexe00)
 | 764
 | 11308
 | Error (cr-restore.c:1280): 11308 exited, status=255
 | Error (cr-restore.c:1412): Someone can't be restored

which is a way better than silent loop.

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>